As known, a bicycle bottom bracket assembly comprises a shaft, two crank arms associated with the opposite ends of the shaft and a pair of rolling bearings (hereinafter also simply indicated as “bearings”), each configured to be mounted on a respective end portion of a shaft-housing box suitably provided in the bicycle frame. Each bearing is intended to be coupled with a respective shaft portion adjacent to a respective crank arm so as to allow the rotation of the shaft inside the aforementioned shaft-housing box.
The shaft can be made in a single piece or in two distinct pieces coupled together, hereinafter indicated as “shaft elements”. A shaft element can be distinct or made in a single piece with one of the two cranks. The assembly of a shaft element and of the respective crank arm is indicated hereinafter as “crank arm assembly”.
The present invention has a preferred application in the case in which the shaft is defined by two shaft elements coupled together.
In many bottom bracket assemblies, the bearings are supported inside or outside the shaft-housing box by respective bearing cups mounted at the opposite end portions of the shaft-housing box. The mounting of the bearings outside the shaft-housing box allows the bearings to be brought closer to the crank arms, thus reducing the bending moment generated at the ends of the shaft. The mounting of the bearings inside the shaft-housing box, on the other hand, allows a shorter shaft and therefore one that is less deformable under torsion to be made.
The bearing cups are typically made of metal or plastic material and can be configured either to be screwed to the end portions of the shaft-housing box provided in the frame of the bicycle or to be coupled with interference with the end portions of the aforementioned shaft-housing box.
U.S. Pat. No. 9,388,847 describes a bottom bracket assembly wherein a provision is used suitable for avoiding having too high interferences. Such a document shows, in FIG. 10, a bush mounted with interference on the shaft-housing box provided on the frame of the bicycle. On the radially inner surface of the bush a bearing-housing seat is provided having a first annular portion designed to be coupled with interference with the bearing. In axially inner position and adjacent to the aforementioned first annular portion a second annular portion is provided having a greater diameter than that of the first annular portion and greater than the radially outer diameter of the bearing. Such a second annular portion is intended to allow possible overpressures to which the bearing is subjected following a too high interference between bush and shaft-housing box to be discharged.
The Applicant has felt the need to identify an alternative solution to that of U.S. Pat. No. 9,388,847 to avoid subjecting the bearing to overpressures due to a too high interference between bearing cup and shaft-housing box, such a solution also being suitable for avoiding running the risk of having a too low interference.